Optical transmission systems employ electromagnetic waves from a spectrum of wavelengths including but not limited to the visible spectrum, and often including infrared. It is to be understood that here, expressions such as light, optical and related terms are not to be restricted to the visible spectrum. In optical communication systems a light beam is modulated in accordance with information to be conveyed, and transmitted along dielectric waveguides to a receiver. Typically, transmission of broadband signal content, such as analog multichannel video, requires the use of narrow linewidth light sources, in conjunction with low loss, single mode optical fibers. A typical transmitter for CATV operates at a wavelength of 1550 nm, and includes a narrow linewidth, continuous wave Distributed FeedBack (DFB) laser and an external modulator.
Optical transmission at 1550 nm has two important advantages. First, the attenuation of standard, single mode optical fiber is at a minimum near 1550 nm. Second, efficient Erbium Doped Fiber Amplifiers (EDFAs) also operate in the 1550 nm wavelength region. EDFAs provide saturated output powers exceeding 20 dBm with minimal distortion when amplifying linear fiber optic signals. By using EDFAs to boost the signal power, the range of the fiber optic links can be greatly extended.
However, there is limited noise introduced onto the signal by each amplifier. To avoid degrading the carrier to noise ratio (CNR), it is desirable to design the system fiber links so as to ensure a high input power to each downstream EDFA. Also, it is desirable to minimize the total number of EDFAs installed in links which require multiple EDFAs. Together, these system design constraints require the launching of high power optical signals into each segment of the fiber optic link.
As is well known in the art, nonlinear optical effects, due to the interaction of the local electric field across the fiber and the fiber material, restrict the optical power that can be launched into a single mode optical fiber. These non-linear effects also depend on the length of the fiber, with a cumulative degradation of performance resulting as the length of the fiber increases. The two most significant nonlinear fiber effects exhibited in silica fibers at high power levels are Self Phase Modulation (SPM) and Stimulated Brillouin Scattering (SBS).
The index of refraction of glass is altered in the presence of signals with high optical power densities. This results in the phenomenon known as Self Phase Modulation wherein the index of refraction of an optical fiber is modulated at frequencies where the optical carrier is amplitude modulated. Thus, when a high power optical signal is launched into an optical fiber, the signal acquires optical phase modulation at all frequencies for which amplitude modulation of the optical signal is present. Depending on the transmission distance and maximum frequency being transmitted, this limits the maximum launch power. For example, for a transmission distance of 65 km, at a maximum frequency of 750 MHz, and a Cumulative Second Order Distortion (CSO) specification of -65 dBc, the maximum launch power would be limited to about +13 dBm (20 mW) , unless a compensation technique for SPM is used.
Stimulated Brillouin Scattering (SBS) occurs when a narrow linewidth optical beam is launched into a low loss optical fiber above a threshold power level. High optical power densities generate acoustic phonons in optical fiber. This results in SBS wherein the phonons produced by a high power optical signal can reflect that signal, resulting in increased attenuation and noise on the optical signal. The amplitude of the reflected energy increases rapidly if the optical power exceeds the SBS threshold. For standard single mode fiber, the SBS threshold is in the range of 6-7 dBm (4-5 mW). SBS can also be characterized by an optical linewidth. For standard single mode fiber, the SBS linewidth is in the range of 20-30 MHz. It is the amplitude of the optical signals within the SBS linewidth that determines the amount of optical scattering.
A conventional approach to increasing the SBS threshold is to spread the transmitted optical power over a wide range of optical wavelengths. As long as the power within the SBS linewidth does not exceed the threshold, SBS will remain adequately suppressed to avoid signal impairments. Prior art attempts to suppress SBS typically have involved the optical broadening of the linewidth of the output of the optical source, either through external phase or frequency modulation. Such approaches, while increasing the SBS threshold, do not adequately address second order non linear effects due to self phase modulation.
For example, the refractive index of optical fiber varies with wavelength so that different wavelengths of light travel at different velocities down a fiber. This effect, known as dispersion, spreads the transmitted signal energy in time. The majority of presently installed fiber links, use fiber with a zero dispersion wavelength near 1310 nm. Thus, at 1310 nm where dispersion is not an issue, the SBS threshold can be increased simply by using an optical source whose linewidth, without modulation, is large compared to the SBS linewidth of the fiber. However, the dispersion at 1550 nm, at about 17 psec/nm-km, is quite high. Therefore, to avoid the negative implications of dispersion, typical external modulation links operating at 1550 nm, utilize continuous wave DFB lasers with optical linewidths less than the SBS linewidth of typical signal mode fiber. Thus, unless some method is used to spread the signal linewidth, the maximum optical power that can be launched is about 6-7 dBm.
Dispersion also creates several complications when an optical source is phase or frequency modulated to broaden its linewidth. For the purpose of this discussion, the terms phase modulation and frequency modulation can be used interchangeably. The amount of frequency modulation is simply the time derivative of the phase modulation. For example, when an optical signal is frequency modulated, or chirped, at a frequency f.sub.mod, the resulting optical spectrum consists of the original optical carrier frequency plus sidebands spaced at multiples of f.sub.mod above and below the original optical carrier frequency. That is, an electrical signal at frequency f.sub.1 will have sidebands at f.sub.1.+-.f.sub.mod. To avoid sideband interference with the transmission of a multichannel signal, no sideband of one carrier can fall on another carrier frequency. Effectively, this means that the modulation frequency must be at least twice the maximum frequency in the multichannel transmission band.
Thus, for a link transmitting an optical signal with a maximum frequency of 860 MHz, the minimum frequency for the optical frequency modulation is 1720 MHz. This constraint applies for both direct modulation of a source laser as well as external modulation of the output of the laser with an electrooptic phase modulator.
An additional important consideration when using optical spreading to avoid SBS is a reduction in the transmission bandwidth as a result of dispersion. Because of dispersion, the different spectral components of the transmitted signal propagate at different velocities. When the difference in the propagation delays for the maximum and minimum optical frequencies becomes comparable to the period of the highest RF frequency being transmitted, the response at the higher RF frequencies will be significantly suppressed. Thus, the overall spectral width must be maintained below the value that results in frequency roll-off. For standard 1310 nm zero dispersion fiber, the tolerable spectral width is inversely dependent on the maximum frequency being transmitted and the fiber length. For a 150 km link operating up to 860 MHz, the maximum spectral width is about 0.2 nm or 25 GHz.
Also, due to small fluctuations in the index of refraction of silica fiber, light is scattered at low power levels and propagates in the fiber. A portion of this light is scattered in such a way as to propagate backwards in the fiber towards the transmitter. Similarly, a portion of this light is in turn scattered a second time from the transmitter back towards the receiver. The power level of the double reflected scattered light at the receiver is relatively low, but is nonetheless sufficient to represent a significant source of noise, referred to as Interferometric Intensity Noise (IIN) . Interferometric noise will be present at any frequency that corresponds to the difference between any two optical frequencies that are present in the optical signal.
It is important that the IIN not fall within the main information band, which is typically 50-860 MHz for multichannel video transmission. Thus, the optical spectrum is preferably broken into bands that are no more than 50 MHz wide in which the bands are separated by at least 860 MHz.
More recently, efforts to suppress SBS have combined high frequency modulation of the drive current of a DFB laser with externally imposed phase modulation of the DFB laser output. A primary disadvantage of these systems is signal distortion caused by Self Phase Modulation (SPM). The direct modulation of the drive current of the laser produces both amplitude and phase modulation. Since the main transmission signal has amplitude modulation, there will be distortion of the main signal due to SPM. Prior art modulation schemes do not address or compensate for this distortion.
The system solution of fiber optic transmission at 1550 nm with EDFAs to boost the signal power is the preferred design for long distance transmission of linear fiber optic signals. Prior art approaches do not adequately address the practical limitations of reducing the non linear effects in optical fiber for the transmission of such a signal. Therefore, it would be advantageous to provide a method and an apparatus to minimize the adverse consequences of fiber non linear effects.